1. Field of the Invention
The present invention relates to a terminating circuit in a battery feed circuit of an electronic exchange system. More particularly, the present invention is directed to a circuit for realizing the terminating impedance of a battery feed circuit, which supplies a direct current to subscriber lines or to trunk lines, by an impedance within a transconductance amplifier.
2. Description of the Related Art
The transconductance amplifier is described in, for example, the "Operational Amplifier Circuit Design Handbook" in the Handbook of Operational Circuit Design, D. F. Staut, edited by Milton Kaufman, 1976, McGraw-Hill, Inc., New York, or in "A High-Voltage IC for a Transformerless Trunk and Subscriber Line Interface," by Dennis W. Aull, IEEE Journal of Solid State Circuits, Vol.SC-16, No. 4, August, 1981.
A prior art battery feed circuit is disclosed in U.S. Pat. No. 4,677,669, issued on June 30, 1987.
To supply a current to subscriber lines or to transit trunk lines in an electronic exchange system, a battery feed circuit must be constructed which has a characteristic having a resistance of about 200 .OMEGA. for a direct current and having a high impedance for a differential alternating current. The battery feed circuit must have an impedance matching the load connected to the subscriber lines or to the transit trunk lines.
A terminating impedance of the lines for an alternating current, from the load side, is generally determined as, for example, 600 .OMEGA.+2 .mu.F or 900 .OMEGA.+2 .mu.F. Conventionally, the terminating impedance is connected between the subscriber lines, i.e., between a tip line terminal and a ring line terminal. The 2 .mu.F capacitor should have a tolerance voltage of -48 V applied between the subscriber lines, and further, should have a high precision capacity satisfying a return loss regulation. Accordingly, a problem arises in the conventional terminating circuit in that the size of the capacitor is large, and the presence of this large size capacitor disadvantageously enlarges the size of the battery feed circuit as a whole.